Smalltalk Best Practice Patterns Volume 1: Coding - Free

More documents

Recommendations

Info

Temporary VariableA Composed Method needs temporary storage.How do you save the value of an expression for later use within a method?A state-less language like FP contains no notion whatever of a variable. If you need the value of anexpression, you evaluate the expression. If you need it in many places in a program, you evaluate itmany places.While the abstract properties of a state-less language might be attractive to a language designer,practical programs use variables to simplify the expression of computation. Each variable hasseveral distinguishing features. The scope of a variable is the textual area within which it can beused. The extent of a variable defines how long its value lasts. The type of a variable is signatureof messages sent to it.Long extent, wide scope, and large type all make variables difficult to manage. If all three factorsare present in many variables, you have a program that can only be understood in its entirety.Limiting the scope, extent, and type of variables wherever possible produces programs that areeasier to understand, modify, and reuse.Another language design decision is whether to require the explicit declaration of variables. Earlylanguages like FORTRAN detected the presence of variable automatically. ALGOL and itsdescendants required explicit declaration. Explicit declaration puts a burden on the programmerwriting the program, but pays off when someone else needs to understand the program. Thepresence and use of variables is often the first place a reader begins.Create variables whose scope and extent is a single method. Declare them just below themethod selector. Assign them a value as soon as the expression is valid.Collecting Temporary Variable saves intermediate results for later use. Caching TemporaryVariable improves performance by saving values. Explaining Temporary Variable improvesreadability by breaking up complex expressions. Reusing Temporary Variable lets you use thevalue of a side-effecting expression more than once in a method.Coding Patterns page 78 of 147 9/30/2006
Collecting Temporary VariableSometimes a Temporary Variable is used to collect intermediate results.How do you gradually collect values to be used later in a method?The right set of enumeration protocol would make this question moot. Inject:into: in particularoften eliminates the need for a temporary variable. Code like:| sum |sum := 0.self children do: [:each | sum := sum + each size].^sumCan be rewritten as:^self childreninject: 0into: [:sum :each | sum + each size]The variety of enumeration strategies in complex programs makes it impossible to generalize theinject:into: idiom. For example, what if you want to merge two collections together so that youhave an element from collection a, and element from collection b, and so on. This would require aspecial enumeration method:^self leftFingerswith: self rightFingersinject: Array newinto: [:sum :eachLeft :eachRight |(sum copyWith: eachLeft) copyWith: eachRight]It is much simpler to create a Stream for the duration of the method:| results |results := Array new writeStream.self leftFingers with: self rightFingers do:[:eachLeft :eachRight | results nextPut: eachLeft; nextPut: eachRight].^results contentsWhen you need to collect or merge objects over a complex enumeration, use a temporaryvariable to hold the collection or merged value.Role Suggesting Temporary Variable Name explains how to name the variable.Coding Patterns page 79 of 147 9/30/2006
Page 1 and 2:
Smalltalk Best Practice PatternsVol
Page 3 and 4:
STATE..............................
Page 5 and 6:
PrefaceThis preface will explain wh
Page 7:
AcknowledgmentsI would like to than
Page 10 and 11:
Many of the patterns tell you what
Page 12 and 13:
day to day in development and docum
Page 14 and 15:
What’s Missing?There are a few co
Page 16 and 17:
AdoptionHow can you learn and use t
Page 18 and 19:
Why Patterns WorkHere is the Big As
Page 20 and 21:
Role of PatternsReadingThese patter
Page 22 and 23:
BehaviorObjects model the world thr
Page 24 and 25:
Composed MethodYou are implementing
Page 26 and 27:
Complete Creation MethodA Composed
Page 28 and 29: ConversionHow do you convert inform
Page 30 and 31: Converter Creation MethodYou need t
Page 32 and 33: Query MethodA Composed Method has h
Page 34 and 35: Execute Around MethodHow do you rep
Page 36 and 37: MessagesMessages are the heartbeat
Page 38 and 39: Choosing MessageYou are using a Mes
Page 40 and 41: Decomposing MessageYou are using a
Page 42 and 43: Intention Revealing SelectorYou may
Page 44 and 45: Integer>>addFloat: aFloat^self asFl
Page 46 and 47: Class: SuperFindingClientsuperclass
Page 48 and 49: Extending SuperYou are using Super.
Page 50 and 51: DelegationA Composed Method needs w
Page 52 and 53: Self DelegationYou are using Delega
Page 54 and 55: Pluggable BehaviorHow can you param
Page 56 and 57: Pluggable BlockYou need complex Plu
Page 58 and 59: marriedMenAndUnmarriedWomen^self ma
Page 60 and 61: Instance VariablesI wrote the secti
Page 62 and 63: Variable StateHow do you represent
Page 64 and 65: Timer>>defaultMillisecondPeriod^100
Page 66 and 67: Default Value MethodYou have a comp
Page 68 and 69: Direct Variable AccessYou need to a
Page 70 and 71: Getting MethodYou are using Lazy In
Page 72 and 73: Collection Accessor MethodYou are u
Page 74 and 75: Enumeration MethodYou are using Ind
Page 76 and 77: Role Suggesting Instance Variable N
Page 80 and 81: Caching Temporary VariableA Perform
Page 82 and 83: Reusing Temporary VariableTemporary
Page 84 and 85: CollectionsThe collection hierarchy
Page 86 and 87: CollectionHow do you represent a on
Page 88 and 89: RunArrayYou are using an OrderedCol
Page 90 and 91: | s |s := Set new.Like an OrderedCo
Page 92 and 93: You may need Double Dispatch for co
Page 94 and 95: DictionaryYou need a Collection tha
Page 96 and 97: SortedCollectionYou have a Collecti
Page 98 and 99: ByteArrayYou need to represent an A
Page 100 and 101: (1 to: anArray size) collect:[:each
Page 102 and 103: IsEmptyHow do you test if a collect
Page 104 and 105: ConcatentationHow do you put two co
Page 106 and 107: DoHow do you execute code for each
Page 108 and 109: Select/RejectHow do you execute cod
Page 110 and 111: Inject:into:You need an Enumeration
Page 112 and 113: Duplicate Removing SetYou have a Co
Page 114 and 115: StackHow do you implement a stack?O
Page 116 and 117: implementation of queues in Pascal
Page 118 and 119: Lookup CacheHow do you optimize com
Page 120 and 121: Parsing StreamHow do you write a si
Page 122 and 123: ClassesThere is probably no coding
Page 124 and 125: Qualified Subclass NameYou have cre
Page 126 and 127: FormattingNo other topic generates
Page 128 and 129:
Type Suggesting Parameter NameYou a
Page 130 and 131:
Rectangular BlockYou are writing an
Page 132 and 133:
Conditional ExpressionHow do you fo
Page 134 and 135:
Simple Enumeration ParameterWhat sh
Page 136 and 137:
self listPane parentcolor: Color bl
Page 138 and 139:
Point>>printOn: aStreamaStreamprint
Page 140 and 141:
Development ExampleIn this chapter,
Page 142 and 143:
Money>>setAmount: aNumber currency:
Page 144 and 145:
MoneySum>>printOn: aStreammonies do
Page 146:
| m1 m2 |m1 := Moneyamount: 5curren
show all

Smalltalk Best Practice Patterns Volume 1: Coding - Free

Create successful ePaper yourself

Delete template?

Save as template?